2 Answers
2

I haven't got a great answer for this, but since no-one else has answered ...

As you mention, for the Sachs-Wolfe effect the $C_{\ell}$ values drop off as approximately $\ell(\ell + 1)$ so plotting $C_{\ell}\ell(\ell + 1)$ on the $y$ axis gives an approximately horizontal line and this makes it easy to see deviations from Sachs-Wolfe behaviour. However I suspect the main reason the graphs are drawn this way is that it nicely highlights the doppler peaks. If you just plotted $C_{\ell}$ you'd need to use a log axis and that would make all the peaks look smaller.

Thanks for your answer. I think you're quite close to the aim of plotting the CMB PS in this way.
–
BagheeraMay 19 '12 at 8:40

4

Well, because we're really plotting the anisotropy i.e. variations. So they're the Fourier modes not of the temperature $T$ itself but its Laplacian over the sphere, $\Delta T$, and the Laplacian has a simple well-defined effect on the component $C_l$ which is multiplied by a spherical harmonic function $Y_{lm}$: it just multiplies the spherical harmonic function by $-l(l+1)$. That's why $l(l+1)$ may be identified with the (minus) Laplacian. It's more natural to insert the Laplacian rather than not to really measure "variations" and to get rid of the huge constant term prop. to $Y_{00}$, too.
–
Luboš MotlMay 19 '12 at 8:48